1. Field of the Invention
The invention relates generally to electronic frequency synthesizer circuits and more specifically to single phase locked loops that use a vector modulator to synthesize a wide range of frequencies in very fine digitally controlled steps.
2. Description of the Prior Art
Conventional phase locked loops (PLLs) are ubiquitous in all kinds of communications and navigation equipment. A single frequency reference provided by a crystal oscillator is conventionally mixed with the output of a divide-by-n digital divider connected to a voltage controlled oscillator (VCO). A phase detector is used to compare the divider output frequency and phase to the reference frequency. An error signal drives the VCO in such a direction that the VCO output settles exactly on the nth multiple of the reference frequency. So the accuracy and stability of the reference frequency is imparted to the VCO output frequency. Different frequencies can be synthesized by changing the divisor n, and/or the reference frequency.
Circuit applications that require a very wide range of frequencies and a very fine step size between synthesized frequencies have traditionally meant that multiple PLLs had to be used and their outputs mixed together to generate sum and difference products. Such complex circuits are expensive to manufacture and not as robust and trouble-free as particular applications demand. For example, satellite communications transceivers need to be able to synthesize many different carrier frequencies in the 1.6 GHz band that are separated by as little as fifty hertz. Such fine granularity is necessary to track microwave radio carriers that are Doppler shifted by varying amounts by the relative velocities of the orbiting satellites and mobile transceivers.